System for the amplification of alternating currents



April 17, 1934. J. M. MILLER ET AL SYSTEM FOR THE AMPLIFICATION OF ALTERNATING CURRENTS Original Filed 001;. 2, 1925 5 Sheets-Sheet l April 17, 1934. J. M. MILLER ET AL SYSTEM FOR THE AMPLIFICATION OF ALTERNATING CURRENTS Original Filed Oct. 2, 1925 3 Sheets-Sheet 2 ATTORNEY Z92; QQZ. Zafd wfi I April 17, 1934. J. M. MILLER ETAL.

SYSTEM FOR THE AMPLIFICATION OF ALTERNATING CURRENTS Original Filed Oct. 2. 1925 3 Sheets-Sheet 3 Fatentecl Apr. 17, 1934- SYSTEM FOR AI\Q?L1F1ICATHOT I OF ALTERNASENG CURREN'ES .l'chn M. Miller, Philadelphia, la., and Louis A. Gebhard, i Washington, l). C.

Serial No. 551,783

1 Grains.

Our invention relates broadly to electron tube systems for the amplification of alternating currents and more particularly to a high frequency signal transmission system.

This application is a continuation of our copending application Serial No. 60,071, which was filed October 2, 1925.

One of the objects of our invention is to provide an amplification system for alternating currents in which alternating currents of high frequency may be amplified to a high degree through a relatively small number of stages of amplification with means for balancing the amplification stages to eliminate the circulation of undesired oscillatory currents therein.

Another object of our invention is to provide amplification system comprising a tandem arrangement of electron tubes of successively higher power for increasing the amplitude of high frequency oscillations to a desired value for high power transmission where the ratio of rated power output of one electron tube with respect to a preceding electron tube is relatively high.

Still another object of our invention is to provide a radio frequency amplification system for increasing the amplitude of high frequency oscillations developed by a relatively weak oscillator where the oscillations are controlled by a piezo electric plate or crystal having constant frequency characteristics whereby energy of a single high frequency may be passed through the amplification system with means for preventing reaction of the amplifying stages with respect to the oscillator.

Still another object of our invention is to provide an electron tube amplification system in which a relatively small number of electron tubes may be employed for producing high frequency oscillations of large amplitude from a constant frequency oscillator of relatively small amplitude where the stages of amplification are balanced in such manner that reaction of the amplification stages with respect to the oscillator is prevented and the oscillations confined to the oscillator circuit. v

Other and further objects of our invention will be understood from the specification hereinafter following by reference to the accompanyrawings, wherein:

Figure 1 illustrates diagranmiatically the application of our invention to a radio frequency amification system at a radio transmitter; 2 is a modified wiring diagram showing connections which may be employed for balancing the circuits of the amplifying stages for eliminating (ill. 179-4511) undesired circulating currents therein; Fig. 3 shows another method of coupling the circuits of the radio frequency power amplification stages at a radio transmit er; Fig. l is a theoretical diagram of one stage of power amplification showing the arrangement of circuits for balancing the amplifier and preventing undesired reaction in the circuits thereof; and Fig. 5 shows a self-oscillatory system for impressing oscillations upon the power amplification system at a radio transmitter.

We have successfully employed the invention herein in a radio transmission system, including a tandem arrangement of three electron tubes for the development of the high frequency signaling energy, in practical radio transmission for distances of five thousand miles and more. The electron tube transmitting system described herein has produced results equivalent to the high power radio transmission installations of the United States Navy which cover large areas of land and require massive machines in their operation. The system of our invention occupies a relatively small space and employs a small antenna as compared to the large tracts of land heretofore required in high power installations. Heretofore in high power installations it has been necessary to erect tall supporting towers, provide a large antenna and ground system and to install massive machines such as a high power arc converter or a high frequency alternator for long distance radio communication. In our present system we provide an electron tube master oscillator, the operation of which is controlled by the oscillations generated by a pieso electric crystal. In order to fully utilize the oscillations developed by the piezo electric crystal it is highly important that the tube oscillator should be relatively Weak. lhe master oscillator preferably has only a few watts of output and yet with the circuit arrange ment of our invention we are enabled to produce an output power of a number of kilowatts with only two amplification stages. Prior to our invention numerous stages of amplification have been required to aifect this result. We secure an output power of a single definite frequency controlled by the frequency of the natural oscillations of a piezo electric crystal. The output of the crystal controlled master oscillator is limited to a few watts, for if a high power tube with high voltage is employed in the attempt to get a high output powe, the crystal will chip or break by reason of the intense mechanical vibrations. These improvements are also of value where an ordinary oscillator electron tube is employed as the master oscillator and a constant frequency is desired, for they prevent variations in the frequency of the oscillations of the master oscillator due to variations in succeeding stages of the amplifier, such as variations in the resistance or reactance of the antenna circuit which is usually in the output circuit of the last tube. The combination of features which permit the attainment of these new results in a system for power amplification are; first, the use of balancing methods to prevent reactions through capacity coupling between the output circuit and input circuit of the amplifying tubes; second, the use of a large stepup in the rated power of successive tubes in the amplifier system; and third, the use of considerable steady negative vcltages on the grids of the intermediate tubes and preferably slight underexcitation of the grids of these tubes.

With respect to the first feature, the reactions between the output circuit and the input circuit can, under certain conditions result in instability of the system and the generation of oscillations not controlled by the master oscillator. This is particularly the case when there is very high power amplification per stage as is attained by the combination of features of the present invention. The use of a balancing method prevents such reactions. Under other conditions it is possible for the reaction of the output circuit to be such as to draw power from the input circuit. This is particularly important at super-high frequencies, (that is frequencies over 500 k. 0.)

since in these frequencies it is necessary to use tuned output circuits to obtain eflicient amplification and the successive output circuits are coupled together through the internal capacity between the grid and plate elements in the tube and other extraneous capacities such as those between leads. At these super-radio frequencies this coupling becomes so effective that when a succeeding circuit is tuned to resonance with a preceding circuit most of the energy in the preceding circuit will be transferred to the succeeding circuit and little will be left to supply the grid circuit of the amplifying tube with the necessary exciting energy. The success of the present invention requires the exciting tube to be of low power compared to the tube being excited. It is necessary, therefore, that the power of the exciting tube be conserved for exciting purposes alone; otherwise the operation will not be successful. The use of the balancing methods also prevents reaction of this type. It is possible of course to obtain conditions intermediate those considered above in which there is neither excessive reaction of one type nor the other, but such adjustments are critical and not desirable from the practical standpoint.

With respect to the second feature, it has been customary heretofore to employ in successive stages of a power amplification system, a tube or tubes in a succeeding stage having a power rating of three to five times the power rating of the tube or tubes in the preceding stage. We preferably employ a ratio far in excess of this, for example, a ratio of thirty times to a hundred and thirty times or more. We have employed successfully an electron tube rate at 7.5 watts to excite an electron tube rated at 1,000 watts.

The third feature also results in a conservation of the power required to excite the input of a given tube in the system and hence assists in obtaining the desired result of very high power amplification per stage. If an electron tube is operated with a considerable negative voltage on the grid, the power required to excite the tube is extremely small, provided the amplitude of the exciting voltage is not excessive. It requires no more power to excite a high power tube somewhat less than normal, than to excite a lower power tube to normal value, yet the output of the higher power tube can far exceed that of the lower power tube. This feature therefore in combination with the other features described above, permits the attainment of the desired result of the high power amplification per stage.

In a practical installation of our invention we have employed a master oscillator of 7.5 watts to excite an intermediate amplification tube of two hundred and fifty watts, the energy from which is in turn impressed upon a tube of 20 kilowatts output, and yet the balancing arrangement interposed between the output and input circuits of the several tubes is such that undesired reactions are eliminated.

Referring to Fig. 1 of the drawings, a practical form of the invention has been illustrated which combines the three features essential to the successful attainment of the desired result. In this embodiment it is important that the oscillations be generated by a crystal controlled elec ron tube 1, amplified in a plurality of stages of amplification formed as by tubes 2 and 3 and fed into the transmitting antenna such as 57.

In Fig. l, the electron tube 1 is a low power tube and generates oscillations, the frequency of which is controlled by the quartz crystal 4. The crystal is shown connected between the filament and grid of the electron tube. The battery 6 supplies a negative voltage to the grid of the electron tube through the choke coil 5. The condenser 53 is a radio frequency by-pass condenser. The filament 10. of the electron tube 1 is heated in a well-known manner with alternating current from the secondary 7 of a transformer having a primary winding 10 connected to leads ll over which the alternating current is supplied. The source of continuous voltage B2 supplies the plate circuits of both the tubes 1 and 2 through leads 26, a reduced voltage being supplied to tube f. through the voltage divider consisting of resistances 27 and 28. The plate current is read on the ammeter In, the plate voltage by the voltmeter Ei. The condenser 31 is a high frequency by-pass condenser. The high frequency output current is generated in the circuit comprising the parallel coil 18 and condenser 20 and is read by the ammeter I10. The particular circuit here described for the generation of oscillations controlled by a piezo electric crystal is described and claimed in a co-pending application by John M. Miller filed September 10, 1925, Serial No. 55,464, which became Patent 1,756,000 April 22, 1930 and is not the subject matter of the present invention.

In the present invention 2 is an electron tube of much higher power rating than tube 1. The exicting voltage is applied to the grid of tube 2 through the connection between coil 18 and the grid which includes the condenser 21 and 21112-- meter 19. The grid is rendered negative by the voltage derived from the battery 133, through the connections 62, running from the junction of resistances 39 and 40 through the signaling he 3", the ammeter Is and choke coil L3 to the grid 2b. When the signaling key 35 is open, the grid connection is made through resistance 38 to the extreme negative terminal of the source B3 which effectively blocks the tube so that it no longer actsas an amplifier. It is essential to this invention, that the negative voltage applied to the grid of the tube 2 be suilicient in actual operation so that when signaling key is closed and the exciting voltage is applied, only a small grid current is registered by the ammeter Is and only a slight reduction in the current is indicated by ammeter I10. The filament 2a is supplied from the secondary 12 of a transformer having primary winding 16 connecting to supply line 17. The plate supply voltage for the tube 2 is furnished by the source B2 through the leads 26, arnmeter Is and choke coil 32. The condenser 22 serves as a blocking condenser for the ccnt uous plate voltage and as a by-pass for high frequency cur rents. The amplified output current of high frequency flows in the circuit comprising the coil 24:, condenser 26 and amineter I4. An essential feature of the present invention consists in the provision of the connection containing the condenser 23 and coil 19 in combination with the other features described. With a suitable number of turns in coil 19 and proper polarity of the tapped connection on this coil and a suitable value of capacity in condenser 23, reaction is prevented between the output circuit and input circuit of the electron tube 2 through the eifective capacity between the plate and grid electrodes of the tube, and other extraneous capacities such as leads, terminals and apparatus associated with the circuits. It will be understood that other methods of preventing such reactions can be employed as illustrated in Figs. 2 and 3.

The tube 2 functions as an amplifier for increasing the amplitude of the radio frequency energy developed by the oscillator 1 impressing the energy thus developed upon the electron t 1b 3. The exciting voltage for the grid of electron tube 3 is supplied through the tapped connection on coil 24 which contains the arnmeter Is. The rated power of tube 3 can be much larger than that of tube 2, but normally the ratio in this case would be less than the ratio of the rated powers of tube 2 to tube 1. For example, if tube 3 is a ZO-kilowatt tube, that is a tube capable of supplying ZO-kilowatts of power to the transmitting antenna, under normal conditions of operation, tube 2 might be a one kilowatt tube and tube 1 a five or ten watt tube. Since tube 3 is the last tube it is advantageous to excite this tube normally, thus a relatively large amount of power is taken from the exciting tube 2. Reactions between the output circuit of tube 3 and the input circuit are prevented by the connection running from the tap 63 on coil 24 and including condenser 52 in a similar manner to that explained before for tube 2. The supply of voltage for the plate of tube 3 is furnished by the source B4, through leads 61, and choke coil 49. 51, 42 and are the customary by-pass condensers. The output circuit of tube 3 includes the coupling coil 54 which is coupled to coil 55 in the antenna system 57 and counterpoise 58. The antenna circuit is tuned by varying the position of the tap on coil 55 and the variable condenser 56. Filament electrode 3a -s heated from a filamerit transformer comprising primary winding 46 and secondary winding 43 supplied by power through leads 4?. A common bus connection 60 is provided for all or the tube circuits as shown. The keying circuit may be distantly controlled through winding 36 which is connected to line wire system 37.

In order to eliminate the undesirable sheets or" the alternating current which is utilized for heating the cathodes 1a, 2a and 3a, we provide a center tap on the secondary windings 43, 12 and 7 of the filament heating transformers. In order to afford a low reactance path for the radio frequency return currents to the cathodes we preferably provide condensers 8, 9, 1e, 15, 4e and l5 connected across each half of the secondary windings 7, 12 and 43 respectively, of the filament heating transformers. We provide resistances in each of the voltmeter circuits. Meter E4 has a resistance 30 disposed in series therewith, while meters E1, E2 and E3 have resistances i8, 41 and 29 disposed in circuit therewith.

A resonance circuit including inductance 59 and annneter I2 is disposed between the output or" tube 3 and the coupling system which connects with the antenna. We may provide a balanced system as shown in Fig. 2 where the balancing inductances and condensers are shown at 64 and 52, also 63 and 23 in a different relationship than that illustrated in connection with Fig. 1. We have illustrated only electron t 2 and 3 with the plate radio frequency by-- condenser 22 and 51 connected in the output circuits thereof. Condensers l4 and 15, also 44 and 45 are the filament radio frequency bypas' condensers. or parallel feed of plate and grid C. voltages be used in any plate or grid circuit but if a series feed plate circuit is the succeeding grid circuit must be parallel feed, or vice versa. Variable condensers are shunted across inductances 2d and .59 providing the resonant circuits for tubes 2 and 3 respectively. The cor denser 23 and 5a are the balancing capacities tubes 2 and 3 respectively. The taps 65 and 56 are located on opposite sides of ground tap 6? on the inductance 59, while taps 68 and 69 are located on opposite sides of the ground connection 70 of the inductance 24. After the 66 and 69 are properly adjusted for correct voltage and resonance circuits 24, I10, and 25, and 59, I4, are properly adjusted to resonance the taps 65 and 68 with condensers 52 and 23 are properly adjusted for a balance.

In Fig. 3 we have shown still another method of balancing the reaction between the tubes where the balance is obtained on tube 2 through capacities 71 and 72 and on tube 3 through capacities '73 and 74. The resonance circuits, plate and grid radio frequency by-pass and blocking condensers are similar to those described in Fi 2.

In Fig. 4 we have illustrated a theoretical diagram of one of the amplification stages showing the method by which the internal capacity of the tube, that is the inherent capacity between the grid and plate electrodes is balanced so that the coupling between preceding and succeeding resonant circuits is reduced to a point where practically no power is transferred from the former to the latter and all of the power is conserved for grid excitation for amplification purposes. The parts comprising a stage of amplification including electron tube 2 of the amplification circuit of Fig. l, have been illustratedv 1 In the resonance circuit 24 and 25 there is a complete balance. There is no tendency for selfoscillation in the circuit for there is no voltage across the resonant circuit and current does not pass. The voltage supply at E is represented as f that voltage due to the current in the resonant circuit 18-20-110 of Fig. l. ince the coupling due to the capacity between the grid and plate elements of the tube, leads connected to elements, and associated apparatus is completely It will be understood that series balanced by the inductance 19 and capacity 23, the resultant voltage across the resonant circuit 24 and 25 due to the voltage supply at E is Zero. All magnetic coupling between successive input and output circuits is eliminated by suitable shielding interposed between these circuits. Therefore there is no tendency for self-oscillation in the tube because the essentials of energy feed between plate and grid circuits of the same tube is lacking.

In Fig. 5 we have shown connections for a balanced amplifier system when using a self-oscillating tube instead of crystal controlled circuit. In this circuit condenser is a grid blocking condenser in the grid circuit of the electron tube 1, while resistance '76 serves as a grid lead. The inductance 18 has taps '77 and 78 on opposite sides of tap '79 so that the voltages impressed upon the grid and plate will be 80 degrees out of phase which is the correct condition for oscillation. Oscillations will set up in the resonance circuit consisting of inductance 18, condenser 20 and ammeter I10. While we have shown one preferred form of self-oscillatory circuit it will be understood that other forms may be used. The inductance 19 is coupled closely with inductance 18 and in fact in this case they are the same coil. Grid excitation for the tube 2 is obtained through blocking condenser 21 and ammeter I9 from tap 80. Balance is obtained through the tap 81, capacity 23, and blocking condenser 22. Taps 80 and 81 are located on opposite sides of the tap 79 so as to afford the proper phase relation. We have shown for purposes of illustration only a relatively small number of stages of amplification in the amplifying system, but it will be understood that any number of stages may be employed. We have also shown an alternating current supply system for the electron tubes, but we may also provide a direct current supply system where desirable without departing from the spirit of the invention.

Our invention is applicable to radio broadcast transmitters used for the transmission of radio news, music, concerts and other entertainment. The crystal controlled circuit enables the frequency of transmission to be maintained constant at a single frequency. This enables broadcasting stations to operate on wave lengths closely related to each other without interference where selective broadcast-receiving apparatus is provided. The small number of tubes required for high power broadcasting when utilizing the principles of our invention considerably facilitates the installation and maintenance of a broadcast station which heretofore has required relatively large number of tubes for its operation.

While we have shown our invention in certain preferred embodiments, we desire that it be understood that modifications may be made and that no limitations upon the invention are intended other than those imposed by the scope of the appended claim.

What is claimed is:

Radio apparatus comprising a pieZo-electrically controlled electron tube oscillation generator, an electron tube amplifier having at least one electron tube whose grid is underexcited and having a relatively high step-up ratio as between .1

the outputs of power from successive stages, and means including balancing devices for preventing capacity coupling between the output and input circuits of the respective amplifier tubes and for the prevention of feed-back to the piezoi electric element of said oscillator generator.

JOHN M. MILLER. LOUIS A. GEBI-IARD. 

